Underpinning measurements of airborne contamination in clean rooms
Airborne molecular contamination in the form of chemical vapours or aerosols has an adverse effect on products, processes or instruments. Technological progress is driven by the ability to operate at ever smaller scales and with greater complexity, which in turn increases the demand for lower concentrations of contaminants. Real‑time, online monitoring is critical to ensure that corrective action is taken before contamination impacts on production costs.
Our research underpins a new generation of ultra‑sensitive spectroscopic techniques for key airborne molecular contaminants, such as HCl and NH3. We are working towards developing high‑accuracy reference materials below 10 nmol mol-1 using dynamic methods to produce dilutions higher than 10,000:1, with a target accuracy better than 0.5 % relative. We are also developing instrumentation and novel passivation techniques to optimise the long‑term stability of the reference materials.
These developments are urgently needed to improve manufacturing processes and the control of contamination. This will increase industrial competitiveness, reduce down time and remove barriers to efficient manufacturing.
Underpinning measurements of water vapour in process gases
Water vapour is one of the most difficult impurities to remove from gases, and it affects a number of manufacturing processes, even at trace amount fractions. Instrumentation dedicated to measuring trace levels of water is therefore of paramount importance. The accuracy of these instruments can only be maintained through regular calibration to traceable reference standards. Reference standards of water vapour provide an accurate method for calibrating instrumentation.
We have developed a system for generating traceable reference standards of water vapour at trace levels between 5 and 2,000 nmol mol-1. It can provide different amount fractions by using continuous accurate measurements of mass loss from a permeation device coupled with a dilution system based on an array of critical flow orifices. An estimated relative expanded uncertainty of 2 % relative has been achieved for most amount fractions generated. We have demonstrated that this new system has excellent comparability with other methods used by national metrology institutes maintaining standards of water vapour.
Underpinning measurements of high performance barrier layers
One of the biggest challenges for emerging technology based on organic electronics and graphene is assuring the lifetime of the product. These materials are highly-sensitive to moisture and oxygen, and metrology to underpin quantitative degradation studies of the active components and performance measurements of encapsulating barrier materials is essential for future success of the technology.
NPL has developed a new approach to measuring the water vapour transmission rate (WVTR) directly, based on cavity ring-down infra-red spectroscopy. The system operates with a dry chamber separated from two wet chambers of known temperature and relative humidity by the barrier material under test. Water vapour permeating through the film is collected by a flow of dry nitrogen and measured. A major source of error when measuring ultra-low levels of water vapour transport is leakage around the perimeter of the sample under test, where it is clamped into the measurement system. Our measurement cell uses a novel design to minimise leakage through the seal and hence to eliminate water ingress to the dry chamber. This approach is a significant advance beyond existing methods and provides accuracy and traceability with a detection limit below 5 x 10-5 g m-2 day-1.
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